drm/i915: fix the wrong latency value while computing wm0

On Ironlake, the LP0 latency is hardcoded and in ns unit, while on
Sandybridge, it comes from a register and with unit 0.1 us. So, fix
the wrong latency value while computing wm0 on Ironlake and Sandybridge.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>